The specification hereafter describes paper webs, but webs of other materials are also contemplated. In order to improve the printability of paper web surfaces and to adapt them to specific printing processes, such paper web surfaces are at present coated with pigment/vehicle dispersions. Depending on the purpose of use, the dry application weights of the coating materials are between 5 and 35 g./m..sup.2. The coatings are customarily effected using an aqueous dispersion with different contents of solids so that the wet application weights of the coatings are considerably higher than their dry weights.
It has been found advisable to first apply excess coating material to the traveling web of material and to then remove the excess from the surface by dosaging devices. Simultaneous smoothing of the surface of the coating is desired. Known dosaging devices include roller coating mechanisms in combination with dosaging rollers, so-called reserve roll coaters, roll-scraper dosaging devices, which are also known as doctors, and smooth-scraper devices, better known as "blades".
During the dosaging process, the coated web of material is supported by a supporting device, for instance a roller. Pressure is exerted on the web by the dosaging device, for instance a blade or doctor. The pressure acts against the coating composition.
West German Provisional Patent (Auslegeschrift) No. 10 71 039 describes an apparatus for regulating the applied weight of coating material. The web is supported on the uncoated side thereof by a supporting roller, known as a coating drum, while a doctor knife arranged over the coated side of the roller is provided to adjust the applied weight. The doctor knife is arranged on a rotatably supported shaft and can be pressed with variable application pressure against the traveling web.
West German Pat. No. 486,149 describes an apparatus having a stripping knife as the dosaging device, in which the active edge of the knife is rounded.
U.S. Pat. No. 2,051,403 discloses a coating device which is a development of a stripper. A specially shaped stripper is provided having a curvature with two different radii. The end of the inlet flank of the stripper does not have a sharp edge, since such an edge would lead to a defective surface and unsatisfactory results.
Depending on the amount of the pressure exerted by the dosaging device and depending upon the position of that device with respect to the coated web, a thicker or thinner layer of coating material remains on the web. This is also true of known roll-coating and dosaging systems. In this case, the thickness of application of coating material is dependent upon the diameter of the rolls, the hardness of the surface of the rolls and the circumferential speed of the rolls.
Scrapers of the blade edge and doctor bar type are, considered in themselves, not sufficiently stable tools and require mounts and guides impart to them the required linearity, resistance to bending and resistance to the opposing pressure of the coating composition that is moving along with the traveling web. Particularly high demands are made on the development of such mounts. With web widths of several meters and web speeds of up to 1200 m./min., which are customary today, the requirements can be satisfied only to a certain extent by corresponding structural developments. As a result, known dosaging devices are not satisfactory, particularly in the situation where higher coating material application weights are sought and only low pressures may be opposed to the stream of coating material. Even slight deviations in the linearity of the dosaging device along the direction transverse to the web, caused by manufacturing tolerances or other circumstances, lead to considerable variations in the amounts of coating material that are applied to the web after the dosaging, as measured transversely across the web. It has been attempted to improve the scraper linearity by, for instance, supporting the edge of the blade by means of pressure hoses. The results are unsatisfactory, particularly in the case of larger web widths.
In the roll devices, the rolls used can be adjusted with the required accuracy, even in the case of large width rolls, but the surface quality thereby obtained frequency does not satisfy the requirements, since later contact with the surface of the coated material by the roll, after passage of the web through the zone with the highest pressure, is unavoidable. This later contact splits the coating with a resultant greater or lesser reduction of the smoothness of the surface. Depending on the doctor diameter, this effect is also present in the doctor type dosaging devices. Contrary to this, blades have a sharp edge which limits the length of the coating zone, and this avoids subsequent contact by the dosaging device on the coating material.
Depending upon the applied weight of the coating material, the speed of the web and the flow properties of the coating material, one or the other of the known dosaging devices has been found to be more suitable for a particular application. In practice, roller dosagings have proven better with higher weights of coating material, while with lower weights of coating material and high speeds of web, smoothing-scraper devices are preferred. When a particular dosaging system for a coating plant is selected, a decision is thus indirectly also made as to the spectrum of products which can be optimally produced. With the increasing sizes of manufacturing plants and their higher investment costs, upon the installation of coating devices in paper producing systems, the decisions that have had to be made at the outset have proven disadvantageous since the continuous adaptation to satisfy the product requirements of various customers for coated papers cannot be made with single-purpose dosaging system.
The coating plants now used can have operating widths of up to 6000 mm. The speeds of operation and the application weights of the coating materials depend greatly upon the types and the desired properties of the paper to be coated and of the coated paper to be produced. For LWC paper (LWC stands for "lightweight coated"), for instance, the speeds are to up to 1200 m./min. with coating material application weights of up to 10 g./m..sup.2 side. For cardboard, for instance, the speeds are up to 600 m./min., with application weights of about 25 g./m..sup.2.
There has thus been an urgent need in coating plants for dosaging devices for the coating of webs, with which both lower and higher weights of application of coating material can be obtained without reequipping the plant and where both low and high speeds of operation can be used, all without sacrificing the quality of the properties of the coated surface.